1. Field of the Invention
The present invention relates to a pseudo-echo cancellation processing apparatus, a pseudo-echo cancellation method adopted in the pseudo-echo cancellation processing apparatus and a digital-broadcast receiving apparatus employing the pseudo-echo cancellation processing apparatus.
2. Description of the Related Art
In recent years, in the field of DTVs (Digital Televisions), research on high-performance DTV receivers and high-performance digital set top boxes are carried out intensively and extensively. US digital broadcasting ATSC (Advanced Television Sub Committee) specifications prescribe a VSB (Vestigial Side Band) signal for DTV signal transmissions.
As shown in a diagram of FIG. 1, a VSB data frame transmitted to a DTV includes two fields. One of the two fields has 313 segments. The first segment of each of the fields is a field synchronization signal. The field synchronization signal has 832 symbols. FIG. 2 is a diagram showing the field synchronization signal including first four symbols serving as a segment synchronization signal of the field synchronization signal. As shown in the diagram of FIG. 2, the four first consecutive symbols have four levels, i.e., +5, −5, −5 and +5 levels respectively.
The segment synchronization signal is a signal identifying the beginning of the first one of the 313 segments composing a field. Since the first segment serving as the field synchronization signal has 832 symbol and the segment synchronization signal of the first segment includes four symbols, the number of remaining symbols in the first segment is 828. As a matter of fact, the number of remaining symbols in every segment is also 828 as shown in the diagram of FIG. 1.
The field synchronization signal shown in the diagram of FIG. 2 represents the start point of a data field which includes data+(and) an FEC (Forward Error Correction) as shown in the diagram of FIG. 1. The field synchronization signal also functions as a reference signal of a channel equalizer. In addition, the field synchronization signal also functions as a reference signal for determining whether or not to use an NRF (NTSC Rejection Filter).
On top of that, the field synchronization signal is also used for confirming a channel characteristic and for determining a loop performance of a phase tracer in some cases.
The concrete structure of the field synchronization signal is shown in the diagram of FIG. 2. As shown in the diagram of this figure, the field synchronization signal includes sequences such as a PN511 sequence, a PN63 sequence, a VSB mode and a reserved sequence where PN is an abbreviation for Pseudo random Number. Having a length of 511 symbols, the PN511 sequence is used as a training sequence for a channel equalizer.
Much like the PN511 sequence, the PN63 sequence is also used as a training sequence for a channel equalizer. The field synchronization signal includes a total of three PN63 sequences. A specific one of the three PN63 sequences is used for determining whether the field synchronization signal including the PN63 sequences is the field synchronization signal of the first or second fields. That is to say, the sign of the specific PN63 sequence included in the first field is the inversion of the sign of the specific PN63 sequence included in the second field. In this way, the first and second fields can be distinguished from each other.
Having a length of 24 symbols, the VSB mode is a sequence representing the transmission data presently being transmitted. That is to say, the VSB mode is used for indicating whether the data transmission system is a 15VSB system or an 8VSB system. The reserved sequence is a free area having a size of 104 symbols.
If a ground wave broadcast signal with a VSB format having one carrier is used, it is difficult for a DTV receiver to receive the ground wave broadcast signal transmitted through multipath. Thus, a firm signal receiving system is absolutely required as a system capable of receiving such a ground wave broadcast signal since it is difficult for a DTV receiver to receive a ground wave broadcast signal. In order to receive a VSB signal, it is necessary to receive its field synchronization signal or its segment synchronization signal with a high degree of accuracy. There have been proposed methods for receiving synchronization signals with a high degree of accuracy as disclosed in Japanese Patent Laid-Open No. 2004-242325, Japanese Patent Laid-Open No. 2005-333300 and Japanese Patent Laid-Open No. 2006-203485.